Crack propagation and cleavage initiation in Fe-2.6%-Si single crystals under controlled plastic crack tip opening rate in various gaseous environments

Abstract

Crack propagation and crack initiation measurements were carried out in Fe-2.6%-Si single crystals under a new type of control: The elastic strain rate is controlled before yielding. Beyond the yield point, however, the rate of plastic elongation of the specimens is non-linearily controlled. Since plasticity is exclusively concentrated onto the crack tip. the plastic crack tip opening rate is prescribed with reasonable accuracy in this type of experiment. In b.c.c. FeSi single crystals at temperatures below 100°C the crack tip opening rate determines whether the crack advance is ductile or brittle. The critical crack tip opening rate, at which the fracture mode changes, is strongly temperature dependent and yields an activation energy of 0.68 eV for the initiation of cleavage. This value is close to the activation energy reported in the literature for dislocation glide in FeSi. In a small range of the crack tip opening rate, below the critical rate, stable brittle fracture (‘quasi-brittle fracture’) is found. Evidence is reported that plasticity may be responsible in a yet unknown way for the stable quasi-brittle crack growth.